Heat resistance high enough to withstand the heat applied during the soldering process is one of the essential requirements for insulating materials used in various electronic devices, such as substrates for flexible printed wiring circuits, substrates for tape automated bonding, protective films for semiconductor devices, and interlayer insulating films for integrated circuits. Polyimides are heat-resistant insulating materials that meet this requirement and are thus widely used in electronic devices.
As insulating materials have become increasingly used in an ever-broader range of applications, other properties than high heat resistance are required of these materials, including low dielectric constant, low thermal expansion, high transparency, high frequency characteristics (low dielectric loss tangent), low moisture absorbance, high dimensional stability, adhesion, workability, and the like. No known polyimides, insulating materials now commonly used in electronic devices, have ever met all of these requirements. Thus, much effort has been devoted to developing non-polyimide heat resistant polymers and composite polyimides in which polyimides combined with other polymers are chemically modified.
The high heat resistance of polyimides results from their rigid backbones and restricted intramolecular rotation. Polyazomethines made by polycondensation of dialdehyde and diamine are also heat-resistant polymers that have similar rigid backbones. The compounds have attracted much attention as a new heat-resistant material.
One drawback of polyazomethines, however, is that polyazomethines with a high degree of polymerization are difficult to obtain because the polymers tend to precipitate at an early stage of polycondensation due to their rigid structure, resulting in the formation of polymers with a low degree of polymerization (See, Non-Patent Document 1). Polyazomethine polymer chains with a low degree of polymerization do not tangle with each other, so that when such a polymer is cast into a film, the film has a decreased toughness and become susceptible to cracking. For this reason, few studies have ever reported the properties of polyazomethine films.
One approach to increase the degree of polymerization of polyazomethine is the use of a fluorinated monomer or a bent monomer in the synthesis of polyazomethine. Addition of these monomers serves to decrease the intermolecular force of the polymer and, thus, increase the solubility of polyazomethine in the polymerization system (See, Non-Patent Document 2, Patent Document 1 and Patent Document 2). It is expected that this approach decreases the precipitation of polyazomethine at an early stage of polymerization.
In recent years, reducing the thermal expansion of heat-resistant insulating films has become an important issue for the following reasons. For example, when a polyimide insoluble in common ordinary solvents is used to make a polyimide film, a soluble precursor of the polyimide is first dissolved in an amide organic solvent and the solution is applied to a metal substrate, which in turn is dried and subjected to thermal dehydration ring closure reaction (imidization reaction) at 250° C. to 350° C. to form a polyimide film. As the polyimide/metal substrate laminate is cooled from the imidization temperature to room temperature, thermal stress is generated, causing the film to curl, peel or crack. Even if the film does not peel or crack, the residual stress significantly decreases the reliability of devices, such as multilayer wiring boards that are increasingly used today as electrical circuits become highly integrated. The stress generated during the imidization process tends to increase when the difference in the linear thermal expansion coefficient between the metal substrate and the polyimide film is large or when the imidization temperature is high. Thus, it is important to minimize the thermal expansion of heat-resistant insulating films.
With regard to reducing the thermal expansion of polyimides, common polyimides have a linear thermal expansion coefficient of 50 to 90 ppm/K, which is significantly larger than that of metal substrates, for example, copper substrates have a linear thermal expansion coefficient of 17 ppm/K. Therefore, studies have been conducted in an effort to decrease the thermal expansion coefficient of polyimides to a value close to the linear thermal expansion coefficient of copper, e.g., 20 ppm/K or less (Non-Patent Document 3). One such study suggested that polyimides with a low thermal expansion must have a linear backbone structure and its internal rotation must be restricted to make the molecule rigid.
One of the most well-known among the practical low thermal expansion polyimide materials is a polyimide made from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and paraphenylenediamine. This polyimide film has an extremely low linear thermal expansion coefficient of 5 to 10 ppm/K while the value may vary depending on the film thickness and the conditions for making the film (Non-Patent Document 4).
Several other polyimide systems showing a low thermal expansion property are known, each of which has a linear, rigid backbone. For example, in addition to the 3,3′,4,4′-biphenyltetracarboxylic dianhydride, pyromellitic dianhydride and 1,2,3,4-cyclobutanetetracarboxylic dianhydride may be used as the tetracarboxylic acid dianhydride, while in addition to the p-phenylenediamine, 2,2′-bis(trifluoromethyl)benzidine, trans-1,4-cyclohexanediamine, o-tolidine and m-tolidine may be used as the diamine. These tetracarboxylic acid dianhydrides and the diamines may be used in combination to make polyimide systems having a low thermal expansion coefficient.
The exhibition of low thermal expansion coefficient by these polyimide films is based upon imidization-induced spontaneous in-plane orientation. Specifically, when a polyimide precursor is cast onto a substrate, the initially low degree of in-plane orientation of the molecules increases rapidly during the thermal imidization (See, Non-Patent Document 4).
Highly linear and rigid backbones are also essential for polyazomethine systems to exhibit a low thermal expansion property. However, the combination of terephthalaldehyde and p-phenylenediamine results in the polymer product precipitating at an early stage of polymerization, as described above, making it difficult to obtain the polymer with a high degree of polymerization.
2,2′-bis(trifluoromethyl)benzidine, a rigid fluorinated diamine represented by the following formula (d), can be reacted with a terephthalaldehyde in m-cresol to obtain a polyazomethine high polymer (See, Non-Patent Document 5). However, the film obtained by casting the polymer is whitish opaque and is extremely brittle.

Bent amines as shown by the following formulas (e) and (f) may be added to the reaction system in amounts small enough not to affect the linearity and rigidity of the polymer backbone. The presence of these amines in the polyazomethine copolymer significantly increases the toughness of the resulting polyazomethine cast film (See, Non-Patent Document 5).

However, the rigid polyazomethine copolymer film obtained by using 2,2′-bis(trifluoromethyl)benzidine of the formula (d) as the diamine component and a terephthalaldehyde has a high linear expansion coefficient of 90 ppm/K, failing to achieve the desired low thermal expansion property (Non-Patent Document 5). This means that evaporating the solvent during the solution casting alone is not enough to induce high in-plane orientation of the molecules even if the polymer backbone is linear and rigid.
In recent years, much effort has been put into the study and development of photosensitive polyimides (or precursor polymers thereof) that can significantly shorten the time required for the fine patterning of polyimide films. If the photosensitivity can be added to the polyimide systems that already have properties not seen in common polyimides, such as low dielectric constant, low thermal expansion and high glass transition point, such polyimide systems should serve as a highly useful material in the relevant industrial fields.
As environmental issues become a greater concern, needs are shifting from negative photosensitive polyimide precursors, which are developed by organic solvents, to positive photosensitive polyimide precursors, which are developed by alkaline solutions. Although polyimide precursors (i.e., polyamic acids) are by nature soluble in alkali solutions, they can be made insoluble in alkaline solutions by dispersing a diazonaphthoquinone photosensitizer (which serves as a solubilization suppressant) in the polyamic acid film. When the alkali-insoluble polyimide precursor is exposed to UV rays via a photomask, the diazonaphthoquinone photosensitizer undergoes a photoreaction and is converted to an alkali-soluble indene carboxylic acid in the exposed area. In this manner, only the exposed area is made soluble in aqueous alkali solutions, making the positive patterning possible.
However, the solubility of polyamic acid in an aqueous tetramethylammonium hydroxide solution, a common alkali developer for semiconductor photoresists, is so high that the effect of the solubilization suppressant becomes insufficient. In many cases, this makes the formation of sharp patterns difficult. For this reason, it is necessary to chemically modify the structure of polyamic acid to control its solubility in aqueous alkali solutions.
In addition to controlling the solubility of polyamic acid, the transparency of the polyamic acid film is also important. When the polyamic film is exposed to the i-ray (365 nm) of a high pressure mercury lamp, the polyamic acid film needs to have a sufficiently high transmittance to the wavelength of i-ray. Otherwise, the irradiation is blocked by polyamic acid and does not reach the photosensitizer, so that the exposure takes undesirably long or, in an extreme case, the photoreaction of the photosensitizer is interrupted, resulting in the failure of patterning.
As described above, polyimide films made from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and p-phenylenediamine are well known as low thermal expansion polyimides. However, the precursor polyamic acid film blocks the irradiation and shows virtually 0% transmittance to the i-ray. Thus, the polyamic acid film is unsuitable for use in photopatterning.
In contrast, polyamic acid films made from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and trans-1,4-cyclohexanediamine have a very high i-ray transmittance (See, Patent Document 3). In addition, the polyimide films obtained by curing these polyamic acid films exhibit different properties in a well-balanced manner, including low thermal expansion, low dielectric constant, and high glass transition temperature (See, Non-Patent Documents 6 and 7). However, these polyimide precursors also have an excessively high solubility in aqueous alkali solutions and are unsuitable for use in photopatterning. Thus, practical techniques are sought to make these precursors suitable for photopatterning.
Likewise, practical approaches are needed to impart to the highly heat-resistant polyazomethine systems such favorable properties as low thermal expansion, low dielectric constant, and high glass transition temperature, in a well-balanced manner, and thereby make the polyazomethine systems suitable for use in photopatterning.
Non-Patent Document 1 Yuki Kagaku Gosei (Synthetic Organic Chemistry), vol. 41, 1983, pp. 972-984
Non-Patent Document 2 Macromolecular Chemistry and Physics, vol. 195, 1994, pp. 1877-1889
Non-Patent Document 3 Polymer, vol. 28, 1987, pp. 2282-2288
Non-Patent Document 4 Macromolecules, vol. 29, 1996, pp. 7897-7909
Non-Patent Document 5 Preprint of the Annual Conference of the Society of Polymer Science, Japan, vol. 52, No. 6, 1996, p. 1295
Non-Patent Document 6 High Performance Polymers, vol. 13, 2001, pp. S93-S106
Non-Patent Document 7 High Performance Polymers, vol. 15, 2003, pp. 47-64
Patent Document 1 Japanese Patent Application Laid-Open No. Sho 64-79233
Patent Document 2 Japanese Patent Application Laid-Open No. Hei 2-42372
Patent Document 3 Japanese Patent Application Laid-Open No. 2002-161136